Devices based on the blowing of a gas are commonly employed in the manufacture and refining of paper. In such devices, the gas that is blown is passed by means of various nozzle arrangements to one side or both sides of the web. Thereafter, the gas is sucked off for renewed use or for removal, and/or the gas is discharged to the sides of the web.
Prior art devices based on contact-free treatment of a web consist of a number of nozzle boxes, out of which a gas flow is applied to the web to support and dry the web. The prior art nozzles in these devices can be divided into two groups: nozzles with pressure and nozzle with negative pressure. The operation of the pressure nozzle is based on the principle of air cushioning, whereas the nozzles with negative pressure produce a dynamic field of negative pressure and their carrier face attracts the web and stabilizes the run of the web. As is known in the art, the attractive force applied to the web is based on a gas flow field parallel to the web. The gas flow field forms a dynamic negative pressure between the web and the carrier face of the nozzle. Both in the pressure nozzles and in the nozzles with negative pressure, the so-called Coanda effect is commonly utilized to guide the air flow in the desired direction.
In prior art pressure nozzles, an area with positive pressure is formed between the web and the carrier face of the nozzle. The positive pressure attempts to push the web apart from the nozzle as is shown in FIG. B1. Thus, when nozzles with negative pressure are placed at both sides of the web, the pushing forces of the pressure nozzles compensate for each other and the web runs approximately in the middle. The pushing force, i.e. repulsion, applied to the web at a pressure nozzle is generally at all distances higher than, or equal to, 0. This is evident from FIG. B2 where the pushing force produced by a prior art pressure nozzle and applied to a web as a function of the distance between the web and the nozzle is illustrated.
The force applied by pressure nozzles to a web is relatively high. Thus, by means of pressure nozzles, it is possible to treat heavy and fully non-stretching webs. However, most of the prior art nozzles with positive pressure apply sharp jets in a substantially perpendicular direction to the web. As a result, an uneven distribution of the heat transfer coefficient in the longitudinal direction is produced. This uneven distribution frequently causes damage to the quality of the web that is being treated.
In nozzles with negative pressure, an area with a slight negative pressure is formed between the nozzle and the web. This area stabilizes the web at a certain distance from the carrier face. The formation of the negative pressure results from the mode of blowing of the air, whereby the air jet is guided to run as parallel to the carrier face and web (as seen in FIG. A1). At very short distances between the carrier face of the nozzle and the web, a pushing force, e.g. repulsion, is applied to the web, and at longer distances, an attraction force. FIG. A2 illustrates the attraction/repulsion force applied to a web in connection with a prior art nozzle with negative pressure as a function of the distance between the web and the nozzle.
The force applied to the web by prior art nozzles with negative pressure is relatively low. As a result, these nozzles are, generally, not employed for the treatment of heavy webs of when the tension of the web is low. Thus, nozzles with negative pressure are, generally, employed in devices whose length exceeds 5 meters and in which guide rolls are placed at both sides to support the web.
In respect of the prior art connected with and closely related to the present invention, reference is made to the FI Patent Application Nos. 60,261, 68,723, and 77,708 as well as to the publication by D. W. McLaughlin, I. Greber, The American Society of Mechanical Engineers, Advances in Fluids 1976, "Experiments on the Separation of a Fluid Jet from a Curved Surface", pages 14 to 29. Among these publications, the Finnish patents 60,261 and 77,708 describe pressure nozzles, and Finnish patent 68,723 describes a nozzle for an airborne web dryer by whose means a drying and supporting gas flow with negative pressure is applied to a web to be dried.
In the embodiment described in Finnish patent 68,723, the nozzle slot of the nozzle is placed in the gas flow direction before the level of the inlet edge of the curved guide face. With the occurring gas flow rates, the ratio between the width of the nozzle slot and the curve radius of the guide face can be selected so that the gas flow is separated from the curved guide face substantially before its trailing edge. In this prior art inventions, the nozzle comprises a nozzle box, at one of whose sides there is a nozzle slot. The nozzle slot is defined by the front plate of the flow, on one side, and by the front wall of the nozzle chamber, on the other side and provides a curved flow guide face and a deck part.
The cited paper "Experiments on the Separation of a Fluid Jet from a Curved Surface" examines the mechanisms of separation of a flow jet from a curved wall and the various parameters affecting same. With regard to the present invention, the results that are relevant are illustrated in the graphic presentation in FIG. 5, on page 21 of the above mentioned paper, in which a cluster of curves is shown in a system of coordinates. The vertical axis represents the angle of separation and the horizontal axis represents the Reynolds number. The parameter of the cluster of curves is the ratio W/R=ratio of the width of the nozzle slot to the curve radius of the face. It can be seen from these study results that, with the flow parameters occurring in the nozzle constructions, the follow angle .phi. is preferably in the range of about 45.degree. to about 70.degree..